As a base substrate used as a laminate plate for an electric circuit board, various characteristic properties such as a heat resistance, a dimensional stability against heat, a dimensional stability against humidity, an electrical insulating property, a deformation resistance (not easily form distortion, curling, corrugation, etc.), a light weight, etc., are required.
Since a heat resistant fiber sheet is excellent in several points such as a heat resistance, an electrical insulating property, a dimensional stability against heat, a light weight, etc., as compared with papers made from the other materials, recently, the heat resistant fiber sheet is becoming to be utilized as a base substrate for the laminate plate used for the electric circuit board.
For example, a sheet composed of staple fibers of a polymethaphenylene isophthalamide (Conex; made by Teijin Limted) with a pulp made of the polymethaphenylene isophthalamide (Japanese Unexamined Patent Publications No. 2-236907, 2-106840, etc.) and a heat resistant fiber sheet composed of a copolyparaphenylene/3, 4'-oxydiphenylene terephthalamide fiber (Technora; made by Teijin Limited) with an organic resinous binder (Japanese Unexamined Patent Publications No. 1-92233 and 2-47392), etc., were proposed.
However, since the former not only developed a dimensional change by a shrinkage caused by a heat treatment at a high temperature such as 250.degree. C. or higher, but also an equilibrium water content (water content) of the fiber was large as 5 to 6%, and the content of impurity ions is also large, the paper was inferior in the electric insulating property and could not be used as the base substrate for the electrical insulation to which a high reliability is required.
On the other hand, although the latter had the small equilibrium water content and the impurity ion content thereof was also small since it used the organic resin as the binder composition, it had a problem that the binder migrated to the surface and back sides of the paper during the production processes and was unevenly distributed, and as a result, the amount of the binder existing at the middle layer portion of the paper became very small to deteriorate the uniformity thereof in the thickness direction and reliability of the above paper.
In the case of using such a heat resistant fiber sheet as a laminate plate for the base substrate of an electrical circuit board, since the sheet is subject to a prepreg-preparation step in which the sheet is impregnated with a varnish composition containing, for example, an epoxy resin, and dried, and a prepreg-lamination and shaping step. During the steps, the uneven distribution of the organic resinous binder causes the impregnation and adhesion of the varnish composition to become uneven, a portion of the binder to be fused so as to reduce the adhesion of the staple fibers to each other, and thus the sheet per se to be broken. Also, due to the uneven distribution of the resinous binder, the staple fibers unevenly move from each other and thus the distribution of the staple fibers in the sheet becomes uneven. Further, the resultant laminate for the electric circuit boards is sometimes deformed after a solder reflow step at a high temperature.
Also, a sheet obtained by mechanically binding a para-type aromatic polyamide staple fiber (Kevlar; made by Du Pont Co., Ltd.) and microfibers (Kevlar) of fibrilated para-type aromatic polyamide with a meta-type aromatic polyamide fibrids instead of using the organic resin as the binder, was also proposed (Japanese Unexamined Patent publication No. 61-160500 and Japanese Patent Publication No. 5-65640).
Although this sheet was excellent in characteristic properties such as the heat resistance, dimensional stability against heat, dimensional stability against humidity, deformation resistance (distortion, curling, corrugation, etc.), etc., there was a problem of frequently generating inferior products in electrical insulation tests performed under a high humidity, since the used fibrids were the meta-type aromatic polyamide and had a high equilibrium water content and a high impurity ion content.
That is because, since in the laminate for the electrical circuit board produced by using a base substrate having a large water absorption (equilibrium water content), a migration of the contained impurity ions is generated on passing electricity for a long period of time under a high humidity, an inferiority in the electrical insulation occurred and a reliability for a long period of time could not be maintained.
As described above, various heat resistant fiber sheets were proposed, but a paper base substrate for an electrical insulation material low in water absorption and impurity ion content, excellent in electrical insulating property and also uniformity in the thickness direction of the paper, having a good impregnating property to a varnish composition, and also excellent in bonding property among the laminated layers and deformation resistance, could not be realized yet.